Pulsed-neutron nuclear logging tools operate by releasing high energy neutrons, on the order of 14 Mega-electron Volts (MeV), into the borehole and formation. The neutrons create gamma particles or gamma rays (hereafter just gammas) by one of several of interactions with the surrounding atoms: 1) the neutrons inelastically collide with atomic nuclei and thereby create gammas (known as inelastic gammas); and 2) when a neutron loses energy (possibly through the inelastic collisions or by other means) the neutron is captured by the nucleus of a nearby atom and in the process a gamma is emitted (known as a capture gamma). The atoms with which the neutrons collide and/or the neutrons are captured can be determined by the energy (or frequency) of the gammas produced.
The energy of the gammas produced is determined by a scintillation crystal in operational relationship to a photomultiplier tube. In particular, gammas incident upon and/or within the scintillation crystal create flashes of light, with the intensity of each flash of light based on the energy of the gamma. The photomultiplier tube detects the flashes of light and their intensity for analysis. The photomultiplier tube, along with electronics to record the arrivals, may be referred to as a multi-channel pulse-height analyzer or alternatively as a spectrometer. Detected gammas are assigned to multi-channel pulse-height analyzer channels via a linear relationship, Channel=gain×Energy+Channel offset. Deviations from the desired calibration of the multi-channel pulse-height analyzer can occur for a variety of reasons. For example, voltage drifts caused by changes in the electronics with changes in temperature can lead to deviations in both gain and channel offset. Similarly, light output from most scintillation crystals varies with changes in scintillation crystal temperature which, in turn, affects the overall gain of the system. In addition, aging of the photomultiplier cathode can cause the gain to change over time. For some detector systems a phenomenon known as photomultiplier fatigue occurs with large counting rate changes over short intervals of time. It is common for the calibration to change because of some or all of these effects.
Any technique which provides more accurate and/or easier to achieve calibration of a neutron-induced gamma spectroscopy logging tool would provide competitive advantage in the marketplace.